Song notation

How does song notation work? What is a song? What do you need to write
down in oder to notate a song?

As is the case in many creative endeavors, the answer is relative.

If you want to create a lead sheet, you notate the melody, lyrics and chords.
You add some information about the tempo and mood, and that's about it.

But, what happens when you have a budget and players are expensive and
studio time is expensive, and you need to communicate with musicians quickly
and effectively?

Then you need detailed scores.

What if your song is for piano and voice? Then you have a whole history
of song notation to draw on. You can copy scores from great composers of
the past, and you can learn by following in their footsteps.